One problem with these engines is the removal of the nitrogen oxides formed during combustion from the lean exhaust gas. Depending on the operating state of the engine, the nitrogen oxides emitted by these engines comprise from 60 to 95% by volume of nitrogen monoxide. On account of the high oxygen content of the lean exhaust gas, it is very difficult to reduce the nitrogen oxides to nitrogen and thereby to render them harmless. One possible way of removing the nitrogen oxides consists in purifying the exhaust gases with the aid of a nitrogen oxide storage catalyst.
Nitrogen oxide storage catalysts are well known to the person skilled in the art. They contain basic oxides, carbonates or hydroxides of the alkali metals, alkaline-earth metals and/or rare earths as well as a catalytic component, generally platinum, for oxidizing the nitrogen monoxide to nitrogen dioxide during normal operation, i.e. during lean-burn operation, of the engine with a lean air/fuel mix. During lean-burn operation, the nitrogen dioxide which is generated is bound by the basic components of the storage catalyst in the form of nitrates. Since a nitrogen oxide storage catalyst of this type has only a limited storage capacity, it has to be regenerated from time to time, i.e. the stored nitrogen oxides have to be released again and reduced to form nitrogen. This is done by briefly operating the lean-burn engine with a rich air/fuel mix. The required, regular regeneration of the storage catalyst limits the maximum saving on fuel consumption which can be achieved with a lean-burn engine.
The quantity of nitrogen oxides which has been taken up by a storage catalyst is described by what is referred to as the nitrogen oxide filling level, or just filling level for short. The filling level is the ratio of the quantity of nitrogen oxides actually stored to the maximum quantity of nitrogen oxides which can be stored in the catalyst under the prevailing exhaust-gas conditions.
For the reduction in nitrogen oxide emissions which can be achieved with a nitrogen oxide storage catalyst, it is important for the regeneration to be initiated in good time before the storage capacity of the storage catalyst is exceeded. For this purpose, it is customary to define a limit filling level, which is below the storage capacity of the catalyst. The prevailing, current filling level is determined while the storage catalyst is operating. If the current filling level exceeds the limit filling level, regeneration of the storage catalyst is initiated. If the limit filling level is selected to be a low one, the residual emissions of nitrogen oxides which still remain are low, but the fuel consumption is undesirably increased as a result of the frequent regeneration. If the limit filling level is close to the storage capacity, the proportion of nitrogen oxides which cannot be converted rises. Moreover, there is a risk of the defined limit filling level being exceeded more frequently on account of inaccurate determination of the current filling level, which further increases the remaining nitrogen oxide emissions.
The nitrogen oxide filling level which is present during the storage phase is generally determined continuously by integration of the nitrogen oxide mass stored per unit time at each instant. A mathematical model of the storage process is frequently used for this purpose. For example, DE 100 36 453 A1 describes a method for operating a nitrogen oxide storage catalyst of an internal combustion engine, in which, in a first operating phase (storage phase), the nitrogen oxides from the exhaust gas are stored in the storage catalyst and, in a second operating phase (regeneration phase), are released from the storage catalyst. The start of the second operating phase is determined on the basis of a nitrogen oxide filling level which is modeled on the basis of a nitrogen oxide storage model. To allow the start and end of the second operating phase to be determined as accurately and reliably as possible, the nitrogen oxide mass flow is recorded downstream of the storage catalyst and corrected as a function of the recorded value.
In addition to the problem of determining the optimum instant to switch from the storage phase to the regeneration phase, it is also necessary to monitor the increasing storage capacity with increasing operating time (aging) of the storage catalyst.
The aging of the storage catalyst is composed of a temporary component and a permanent component. The temporary component is related to poisoning by the sulfur compounds contained in the exhaust gas. These components, with the basic components of the storage catalyst, form sulfates which compete with the nitrates. The sulfates are significantly more stable than the nitrates and cannot be removed during normal regeneration of the storage catalyst. Therefore, they reduce the nitrate storage capacity to an increasing extent.
However, the sulfate loading of the storage catalyst can be reduced again. This process is known as desulfating. For this purpose, the catalyst has to be heated to approximately 650° C. and the fuel content of the exhaust gas has to be increased (enrichment). The air/fuel ratio λ during desulfating is typically in the range between 0.7 and 0.98.
The permanent aging of a storage catalyst is related to thermal damage to the component of the storage catalyst. Overheating causes the storage materials themselves and also the catalytically active precious metals to sinter together, so that they lose active surface area. This process cannot be reversed.
For use in a motor vehicle, it is necessary to monitor the aging of the storage catalyst in order to initiate desulfating on demand, to switch the vehicle to stoichiometric operation or if appropriate to trigger a signal that the catalyst needs to be replaced. This process is known as OBD (On Board Diagnosis). A suitable method for testing the efficiency of a nitrogen oxide storage catalyst which is arranged in the exhaust section of an internal combustion engine operated with a lean mix is described, for example, in DE 198 23 921 A1. For this purpose, the current storage capacity of the nitrogen oxide storage catalyst is determined, and a defective nitrogen oxide storage catalyst is diagnosed if the capacity drops below a predetermined minimum capacity.